Materials Science in the Electron Microscope
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Materials Science in the Electron Microscope Frances M. Ross, Guest Editor This issue of the MRS Bulletin aims to highlight the innovative and exciting materials science research now being done using in situ electron microscopy. Techniques which combine real-time image acquisition with high spatial resolution have contributed to our understanding of a remarkably diverse range of physical phenomena. The articles in this issue present recent advances in materials science which have been made using the techniques of transmission electron microscopy (TEM), including holography, scanning electron microscopy (SEM), low-energy electron microscopy (LEEM), and high-voltage electron microscopy (HVEM). The idea of carrying out dynamic experiments involving real-time observation of microscopic phenomena has always had an attraction for materials scientists. Ever since the first static images were obtained in the electron microscope, materials scientists have been interested in observing processes in real time: we feel that we obtain a true understanding of a microscopic phenomenon if we can actually watch it taking place. The idea behind "materials science in the electron microscope" is therefore to use the electron microscope— with its unique ability to image subtle changes in a material at or near the atomic level—as a laboratory in which a remarkable variety of experiments can be carried out. In this issue you will read about dynamic experiments in areas such as phase transformations, thin-film growth, and electromigration, which make use of innovative designs for the specimen, the specimen holder, or the microscope itself. These articles speak for themselves in demonstrating the power of real-time analysis in the quantitative exploration of reaction mechanisms. The first transmission electron microscopes operated at low accelerating voltages, up to about 100 kV. This placed a severe limitation on the thickness of foils
MRS BULLETIN/JUNE 1994
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Materials Science in the Electron Microscope
resolution recordings of the process in semiconductor-metal multilayers at an elevated temperature. Bulk processes which are controlled by diffusion may not be correctly modeled in a thin foil; precipitation sequences in thin foils of Al alloys, for example, are known to be different from those occurring in the bulk.11 However, if the system under study is itself composed of thin layers, this limitation of TEM geometry can be transformed into a real advantage. Experiments of this sort were carried out by R. Hull and J.C. Bean, who describe in this issue their studies of the relaxation process of metastable strained GeSi films grown epitaxially on an Si substrate. Careful computer modeling identifies the conditions under which strain relaxation in a thinned TEM foil approaches the case of an infinitely thick substrate. Heating suitable multilayer structures in situ allows misfit dislocations to form, and real-time observations of dislocation dynamics are used to develop a model for the atomic-scale process of dislocation propagation. Th
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